Having changed the scene units and unit scale, how can I change the default size of new objects?
Ok... I just figured it out by myself...

The scale of new objects is related to the scale of the grid within the "overlays" section if the 3d viewport... If I change the scale in there from 1 to 0.01 all new objects get 100 times smaller than originally which is quite the scale I can perfectly work with: new cubes have 20 x 20mm now...

I do not understand why the setting is located at that place but it works as I want it to work...